1. Field of the Invention
The present invention relates to the field of integrated optics, and more specifically to a method of producing an integrated optical circuit using lithographic techniques.
2. Description of Related Art
Integrated optical devices 100 are optical devices that are realized on a substrate. FIG. 1 is a cross section through the substrate of an integrated optical circuit. Referring to FIG. 1, the substrate 101 includes a carrier 101A and a materials stack 102 comprising thin films of dielectric, semiconductor, or polymer materials grown or deposited on one side of the carrier, here called the top side 103. Common substrate carriers include silicon, quartz, and gallium arsenide. Common dielectrics include silica, germanium doped silica, silicon, silicon nitride, silicon oxynitride, silicon oxycarbide, and LiNbO. Common semiconductors include doped silicon, GaAsInP, and other III-V compounds. Channels are formed in the dielectrics and these channels guide light and perform some sort of optical signal processing function.
In an often used approach for the fabrication of optical devices, a photolithographic mask (also called a photomask or simply a mask), containing an image of the optical circuit design, is employed to print the circuit. FIG. 2 shows a cross sectional view of a photoresist covered substrate being exposed to a source of radiation through a mask. Referring to FIG. 2, the circuit is printed by exposing a photoresist 205 covered substrate 201 to a source of irradiance 202 through a mask 203. The mask 203 contains a pattern of the circuit to be printed. The mask receives its name from the fact that the circuit pattern on the mask selectively blocks (masks) at 203A or transmits (unmasks) at 203B the source of irradiance 202. The remaining or transmissive irradiance 204 that reaches the photoresist after passing through the mask defines the optical circuit pattern. The photoresist 205 on the substrate is subsequently chemically developed leaving a patterned copy of the optical circuit image in photoresist on the surface of the substrate. The photoresist pattern can be either positive or negative. A positive image means that photoresist remains after development where irradiance is masked, while a negative image means photoresist remains where irradiance is unmasked. The photoresist polarity (positive or negative) depends on the mask pattern and the type of photoresist used. The photoresist pattern is transferred into the substrate by etching techniques. A general description of techniques for the fabrication of optical integrated circuits can be found in Hiroshi Nishihara et. al. “Optical Integrated Circuits”, McGraw-Hill, N.Y., 1987, and is incorporated herein by reference.
The technique of and the process for exposing a mask image onto a photoresist covered substrate is called, “photolithography”. The three major types of photolithography are contact printing, proximity printing, and projection printing. In contact printing, the mask is placed directly on a photoresist covered substrate. In proximity printing the mask and photoresist covered substrate are separated by a small distance and there is no optical imaging between the two. In projection printing, lens elements or mirrors are used to focus the mask image onto the photoresist covered substrate, which is spaced from the mask by a comparatively large distance. The mask used in projection lithography is commonly called a reticle. (We will use here the terms reticle and mask interchangeably). Projection printing is commonly used in semiconductor fabrication where substrates are called wafers, and many technologies have been developed, including scanners and step and repeat systems, or “steppers”. Steppers project an image only onto a portion of the substrate. The maximum area printed by a stepper in one exposure is called the stepper field. Multiple images of the reticle, or different reticles, are stepped and repeated over the entire substrate area. Reticle images are typically one to ten times (1× to 10×) the size of the image projected onto the substrate, with image reduction provided by the lens system. The enlarged master image on the reticle results in finer resolution on the actual substrate image. Compared with contact or proximity systems, steppers and scanners are equipped to more precisely align a mask image to a specific position on a substrate.
Conventionally, the entire optical circuit image resides on a single mask. Therefore, in a single exposure of the mask, the entire circuit is printed on a portion of the wafer. If the actual circuit to be realized is larger than that which the reticle size can accommodate, then the circuit image must be distributed across several individual reticles. The individual reticle images are then printed sequentially in a mosaic-like approach. Some means for the alignment of the images must be provided since misalignment will compromise performance. Printing reticles in this manner is called “stitching”, or “field stitching”. An example of an optical circuit that stitches together numerous images is disclosed in U.S. Pat. No. 6,517,997 to S. W. Roberts. Technically this is “inter-field stitching” since numerous fields are stitched together in order to realize a circuit that can not be accommodated within a single field. The term “stitching boundary” is used to describe the border separating two exposed images in the photoresist. In U.S. Pat. No. 6,517,997 there are no waveguide channels traversing across any stitching boundary. An example of stitching for electronic integrated circuits is disclosed in U.S. Pat. No. 6,030,752 to H. J. Fulford.
Field stitching using stepper lithography is typically required for conventional planar lightwave circuits that use low core-to-cladding refractive index contrasts (contrasts below 1%), because such devices are invariably larger than the size of a single stepper reticle. The size of an integrated optical circuit is proportionately related to the core-to-cladding refractive index contrast (which we will refer to here simply as “index contrast”). In the last decade, the use of high index contrast materials has enabled the reduction of the size of devices to the point where they can fit entirely within the field of a reticle. For example, articles by Brent E. Little in Optical Fiber Conference (“A VLSI Photonics Platform”, Proceeding of the Optical Fiber Conference, Vol. 2, pp. 444-445, 2003), and in Brent E. Little et al. in Photonics Technology Letters, (“Ultra-Compact Si—SiO2 Micro-Ring Resonator Optical Channel Dropping Filters”, Photonics Technology Letters, Vol. 10, pp. 549-551, 1998), both incorporated herein by reference, describe high index contrast material systems and devices for realizing micro-circuits. U.S. Pat. No. 6,614,977 and U.S. Pat. No. 6,771,868 both to F. Johnson et al. discloses an ideal low loss, high index contrast material system, and is incorporated herein by reference.
Each unique optical circuit design requires a unique mask image. Optical circuit design, mask design, and substrate processing (micro-fabrication) are all interrelated. In optimizing an optical circuit for production, many iterations of the design and micro-fabrication cycle need to be carried out. This incurs a significant cost and time commitment because commercial grade masks are time consuming to produce and expensive. Indeed, the performance of the final optical circuit is usually dictated by the budget used to procure masks, and the time allowed to accomplish a certain number of iterations.
An integrated optical circuit is comprised of a number optical sub-elements and waveguides. The sub-elements and waveguides are defined geometrically and have a number of parameters that affect their performance. During the development phase of an optical circuit, the optical sub-elements must be optimized both individually and as an aggregate within the circuit, in order to optimize the performance of the entire optical circuit. For example, consider an optical circuit that contains three critical sub-elements. We wish to investigate five different designs for each critical element, and we wish to do this in a complete circuit configuration. This would lead to 5×5×5 or 125 unique circuit permutations requiring 125 mask images. Each unique optical circuit, characterized by a unique combination of sub-elements and waveguides, requires its own unique mask image. Even if one parameter on one sub-element is modified, the entire optical circuit requires a new mask image. Reticles are expensive and time consuming to produce. The optimization of an optical circuit therefore can be costly and time consuming since many reticles need to be procured. Methods to reduce the number of reticles without sacrificing on the number of variables have hitherto not been disclosed.
Further, it often occurs that different products require optical circuits that although unique, are nonetheless very similar in the majority of their circuitry and architecture. For instance, a circuit may comprise twenty five optical sub-elements, twenty three of which are identical for each custom circuit. Each of these circuits requires its own reticle or series of reticles. It would be preferable if minor customization could be achieved by only changing portions of a circuit and re-using the common portion, rather than procuring an entirely custom reticle set for each product. No method has hitherto been disclosed which realizes unique circuits without using unique mask sets and no disclosure has proposed one or more drop-in circuits formed from second circuit masks for producing multiple circuit pattern configurations to form a basic optical circuit from a first circuit mask.